If a regular verb has two or more syllables, if the
verb ends in l or r, and if the last syllable is stressed,
double the l or r before you add -ed:

comp�l / compelled;
conf�r / conferred;
contr�l / controlled;
def�r / deferred;
fulf�l / fulfilled;
pref�r / preferred;
prop�l / propelled

If a regular verb has two or more syllables, if the
verb ends in l or r, and if the last syllable is not
stressed, do not double the l or r, before you add -ed:

c�ncel / canceled;
h�nor / honored;
s�ffer / suffered;
tr�vel / traveled

Note that British spelling does not use this rule*.

*(It means the last rule, about the not stressed syllables)